5 ms·

Emulating a BBC Microcomputer in Javascript

62 points by mattgodbolt 12y ago

deleted 12y ago
[deleted]
scotth 12y ago
> Chrome’s v8 at least bails out its JIT process if a function is either too big, or if it has too many entries in the switch statement. Is this true? Anyone have some sense of why?
- unfunco 12y ago
  I remember seeing in documentation somewhere that switch statements with more than 128 cases are not optimised in V8, that might be what OP is referring to; however, I've never seen a 128-case switch statement in production, nor have I seen a developer use that many cases in development, refactoring tends to happen earlier in my experience.
  - mattgodbolt 12y ago
    The error came from the profiler in v8: the main function was tagged as "Could not JIT - too many cases in switch statement" (or similar). I think it's a fair limitation; as you say, more than a few dozen is rare in "real" code - although one might argue this is a pretty common idiom for handling emulation of bytecodes.
  - XaspR8d 12y ago
    > switch statements with more than 128 cases are not optimised in V8 I recently saw this in Petka Antonov's "Optimization Killers"[1], though I've definitely seen it somewhere else as well, because I recall hearing that variables mixed with literals in a switch could trip up the optimizer. [1] https://github.com/petkaantonov/bluebird/wiki/Optimization-killers#4-switch-case https://github.com/petkaantonov/bluebird/wiki/Optimization-k...
awjr 12y ago
I remember going round a friends house and him loading up Elite onto a BBC B and my mind was blown. An exceptional game in 32kb. Would be interesting to see if they get as far as running Elite.
- mattgodbolt 12y ago
  Elite is the default image! Head to http://bbc.godbolt.org/ http://bbc.godbolt.org/ and hit shift-F12 to boot it :)
  - girvo 12y ago
    My MacBook Pro on Mavericks won't let me use Shift + F12 :(
    - bobochan 12y ago
      You can change that. Goto Systems Preferences | Keyboard and then select the Shortcuts pane. Click Mission Control and then un-click the checkbox that binds F12 to Dashboard (which you probably do not use).
      - girvo 12y ago
        Thanks! I do use dashboard (have my Selfoss RSS reader pinned to it) but only with my trackpad swipe-left :)
- louthy 12y ago
  Bear in mind there would have been about 8k for the screen.
danellis 12y ago
This is really interesting to me. I actually started writing my own about three years ago, also called 'jsbeeb', in CoffeeScript. I didn't get around to finishing it, but the 6502 emulation worked, and it has just enough peripheral support to boot through enough of the OS to display the familiar banner: http://i.imgur.com/HjxV3ks.png http://i.imgur.com/HjxV3ks.png
tasty_freeze 12y ago
Being an American, I was never exposed to the BBC machines. I tried running the emulator and entering a small program; alas, I can't figure out how to enter the "=" key, so I had to give up. I couldn't find "+" either. I found the hardest part of writing my own microcomputer emulator in javascript was keyboard mapping. Each browser does things in a somewhat different way, and then there is the inherent tradeoff between emulating the original keyboard layout faithfully vs using a logical layout (where pressing "=" on the PC keyboard results in a "=" event in the emulator). As for the CPU emulation speed, indeed chrome doesn't optimize switch statements greater than 128 entries. I got around this by coding it as "if (opcode < 0x80) switch (opcode) { first 128 cases } else switch (opcode) { other 128 cases } I had tried using a 256-way opcode dispatch table, which was great for chrome, but it hurt firefox performance (which is blazing with the 256-way switch). Having two 128-way switches was pretty good for both browsers.
- mattgodbolt 12y ago
  Thanks!! I haven't really looked deeply into Firefox performance but I'll definitely give the two-way switch statement a go: thanks!
lucaspiller 12y ago
Good read, although I now have a serious case of neck ache. http://i.imgur.com/HaVWV4H.png http://i.imgur.com/HaVWV4H.png
pling 12y ago
Great stuff. I used a BBC Master up until 1999 as a serial terminal for a Sun machine. The thing was an electronics and experimenters dream: well documented, built in decent BASIC and assembler and ports galore plus if you blew it up, £5 was about all it cost to fix it (I.e. just get another one off eBay). Think they go for a lot more than that now.
camperman 12y ago
If you type in: *CAT at the prompt it will show an image listing. *Elite3 will run the game. Star is mapped to ' normally. It's quite sad that I typed the above commands correctly without thinking when I loaded the page :)
- acqq 12y ago
  I can't manage to type the star. FF29, Win, German keyboard.
bencollier49 12y ago
I'd love to see a messaging system set up to treat any of these JS Beebs currently running in any browser worldwide as though they're all on a single Econet.
HackinOut 12y ago
Here is an x86 emulator in Javascript running Linux 2.6.20 for those who might have missed it (2011): http://bellard.org/jslinux/ http://bellard.org/jslinux/ From the awesome Fabrice Bellard (qemu etc...) Edit: Formatting, additional details

62 points by mattgodbolt 12y ago

deleted 12y ago

[deleted]

> Chrome’s v8 at least bails out its JIT process if a function is either too big, or if it has too many entries in the switch statement. Is this true? Anyone have some sense of why?

unfunco 12y ago
I remember seeing in documentation somewhere that switch statements with more than 128 cases are not optimised in V8, that might be what OP is referring to; however, I've never seen a 128-case switch statement in production, nor have I seen a developer use that many cases in development, refactoring tends to happen earlier in my experience.
- mattgodbolt 12y ago
  The error came from the profiler in v8: the main function was tagged as "Could not JIT - too many cases in switch statement" (or similar). I think it's a fair limitation; as you say, more than a few dozen is rare in "real" code - although one might argue this is a pretty common idiom for handling emulation of bytecodes.
- XaspR8d 12y ago
  > switch statements with more than 128 cases are not optimised in V8 I recently saw this in Petka Antonov's "Optimization Killers"[1], though I've definitely seen it somewhere else as well, because I recall hearing that variables mixed with literals in a switch could trip up the optimizer. [1] https://github.com/petkaantonov/bluebird/wiki/Optimization-killers#4-switch-case https://github.com/petkaantonov/bluebird/wiki/Optimization-k...

awjr 12y ago

I remember going round a friends house and him loading up Elite onto a BBC B and my mind was blown. An exceptional game in 32kb. Would be interesting to see if they get as far as running Elite.

mattgodbolt 12y ago
Elite is the default image! Head to http://bbc.godbolt.org/ http://bbc.godbolt.org/ and hit shift-F12 to boot it :)
- girvo 12y ago
  My MacBook Pro on Mavericks won't let me use Shift + F12 :(
  - bobochan 12y ago
    You can change that. Goto Systems Preferences | Keyboard and then select the Shortcuts pane. Click Mission Control and then un-click the checkbox that binds F12 to Dashboard (which you probably do not use).
    - girvo 12y ago
      Thanks! I do use dashboard (have my Selfoss RSS reader pinned to it) but only with my trackpad swipe-left :)
louthy 12y ago
Bear in mind there would have been about 8k for the screen.

danellis 12y ago

This is really interesting to me. I actually started writing my own about three years ago, also called 'jsbeeb', in CoffeeScript. I didn't get around to finishing it, but the 6502 emulation worked, and it has just enough peripheral support to boot through enough of the OS to display the familiar banner: http://i.imgur.com/HjxV3ks.png http://i.imgur.com/HjxV3ks.png

tasty_freeze 12y ago

Being an American, I was never exposed to the BBC machines. I tried running the emulator and entering a small program; alas, I can't figure out how to enter the "=" key, so I had to give up. I couldn't find "+" either. I found the hardest part of writing my own microcomputer emulator in javascript was keyboard mapping. Each browser does things in a somewhat different way, and then there is the inherent tradeoff between emulating the original keyboard layout faithfully vs using a logical layout (where pressing "=" on the PC keyboard results in a "=" event in the emulator). As for the CPU emulation speed, indeed chrome doesn't optimize switch statements greater than 128 entries. I got around this by coding it as "if (opcode < 0x80) switch (opcode) { first 128 cases } else switch (opcode) { other 128 cases } I had tried using a 256-way opcode dispatch table, which was great for chrome, but it hurt firefox performance (which is blazing with the 256-way switch). Having two 128-way switches was pretty good for both browsers.

mattgodbolt 12y ago
Thanks!! I haven't really looked deeply into Firefox performance but I'll definitely give the two-way switch statement a go: thanks!

lucaspiller 12y ago

Good read, although I now have a serious case of neck ache. http://i.imgur.com/HaVWV4H.png http://i.imgur.com/HaVWV4H.png

pling 12y ago

Great stuff. I used a BBC Master up until 1999 as a serial terminal for a Sun machine. The thing was an electronics and experimenters dream: well documented, built in decent BASIC and assembler and ports galore plus if you blew it up, £5 was about all it cost to fix it (I.e. just get another one off eBay). Think they go for a lot more than that now.

camperman 12y ago

If you type in: *CAT at the prompt it will show an image listing. *Elite3 will run the game. Star is mapped to ' normally. It's quite sad that I typed the above commands correctly without thinking when I loaded the page :)

acqq 12y ago
I can't manage to type the star. FF29, Win, German keyboard.

bencollier49 12y ago

I'd love to see a messaging system set up to treat any of these JS Beebs currently running in any browser worldwide as though they're all on a single Econet.

HackinOut 12y ago

Here is an x86 emulator in Javascript running Linux 2.6.20 for those who might have missed it (2011): http://bellard.org/jslinux/ http://bellard.org/jslinux/ From the awesome Fabrice Bellard (qemu etc...) Edit: Formatting, additional details